US6963683B2 - System and method for a packaging a monitor photodiode with a laser in an optical subassembly - Google Patents

Abstract

A system and method of monitoring a laser output power and laser extinction ratio includes an optical subassembly physically placed between a point light source and a optical fiber device. The optical subassembly creates a convergent light beam by reflecting a collimated light beam from the point light source off an interior surface of a first side of the optical subassembly. The optical subassembly creates an incident ray of the convergent light beam by including on a second side of the optical subassembly a wedge-shaped air gap.

Description

BACKGROUND

1. Technical Field

The system and method described herein relates to monitoring the average optical power output and the extinction ratio of a point light source.

2. Discussion of the Related Art

Optical telecommunication systems include the use of point light sources, e.g., lasers, to transmit information at high speeds through optical fibers. The threshold current and slope efficiency of typical point light sources vary due to age and changes in operating temperature. In order to control the average optical power output of the point light sources, photodetectors are placed in a control feedback loop to monitor the optical output of the laser. If the signal received by the photodiode should fall, for example, the electrical current supplied to the laser would be increased to compensate.

Point light source and monitor photodetector combinations may be mounted in a specially designed package which has a mounting base with insulated connector leads and a sealed cover. The cover may include a window of glass, or other transparent material over a central portion of the top such that the window is aligned with the emitting aperture of the point light source device. In some point light source and photodetector combinations, reflected light from the window of the glass is received by the photodetector. Because the light fluence or power in these systems is generally small and unfocused, large photodiodes are needed to gather enough light to provide a sufficient signal-to-noise ratio (SNR) to maintain the constant average optical output from the laser. Unfortunately, large area detectors have low electrical bandwidth, making them unsuitable for tracking the high speed modulation of the laser. Instead, they are limited to use as time-average power monitors.

Changes in the slope efficiency of the laser with temperature and age also affect the extinction ratio of the point light source output. The extinction ratio of a point light source is the optical power of the “one” state divided by the optical power of the zero state. In systems employing large area monitor photodetectors, the change in extinction ratio is generally ignored or corrected using a look-up table based on data obtained by characterizing lasers similar to those used in the system of interest. Alternatively, the superposition of a pilot tone, at a frequency within the bandwidth of the monitor photodetector, onto the data may be used to correct changes in the extinction ratio of the point light source. This approach, based on the principle that the amplitude of the received pilot tone is proportional to the amplitude of the data modulation, has the drawback of modulating the extinction ratio of the transmitted data as well, thereby introducing extra noise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical communications system according to an embodiment of the present invention;

FIG. 2 illustrates an optical subassembly according to an embodiment of the present invention;

FIG. 3 illustrates a connection of a side of an optical subassembly to a receiving optical fiber device according to an embodiment of the present invention;

FIG. 4 illustrates a path of an incident ray of a reflected light wave and a path of a reflected incident wave according to an embodiment of the present invention;

FIG. 5 illustrates a multi-channel wavelength division multiplexer with fiber optic input according to embodiments of an optical communication system; and

FIG. 6 shows a complete transceiver module according to an embodiment of the present invention.

DETAILED DESCRIPTION

A present embodiment provides a system and method for packaging a monitor photodetector with a point light source in an optical communication system such that a portion of light from the point source is reflected and focused onto the monitor photodetector.FIG. 1 illustrates an optical communications system according to an embodiment of the monitor photodetector packaged with a point source. Theoptical communication system2 may include a point light source3, aphotodetector4, a pointlight source driver5, anoptical subassembly6, a receivingoptical fiber device7, and anoutput port8.

The point light source3 may be a laser. In one embodiment of the present invention, the point light source3 may be a vertical cavity surface emitting laser (VCSEL). The pointlight source driver5 may transmit an electrical signal, such as a current signal, to the point light source3 to provide power for the point light source3. The pointlight source driver5 and the point light source3 may be placed in a closed feedback loop with aphotodetector4 to allow thephotodetector4 to monitor the average output power and the extinction ratio of the point light source3. The pointlight source driver5, the point light source3, and thephotodetector4 may all reside on a common substrate. In another embodiment, the pointlight source driver5 may reside on a separate substrate from the point light source3 and thephotodetector4.

Theoptical subassembly6 may be physically placed between the light point source3 and the receivingoptical fiber device7. In one embodiment of the present invention, theoptical subassembly6 may be composed of optically transparent plastic. In alternative embodiments, the optical subassembly may be made of a polycarbonate, such as LEXAN™, or a polyetherimide, such as ULTEM™. Theoptical subassembly6 may assist in aligning a point light beam from the point light source3 to the receivingoptical fiber device7. In addition, theoptical subassembly6 may, where it is attached to the receivingoptical fiber device7, include a wedge-shaped air gap to create a reflected incident ray of the point light beam. The incident ray of the point light beam may be transmitted back to aphotodetector4. The incident ray of the point light beam may be focused by a lens and thereby received by thephotodetector4 at a relatively large fluence, allowing asmall area photodetector4 to simultaneously track changes in the extinction ratio as welt as changes in the average optical power over time. In one embodiment of the present invention, the lens used to focus the incident ray is the same as the lens used to focus the point light beam onto the core of theoptical fiber device7.

Thephotodetector4 may receive the incident ray of the light point beam producing a photocurrent that is proportional to incident optical power, i.e., watts. The photocurrent modulation amplitude and average value may provide feedback information to the pointlight source driver5 which changes its output signal to the point light source3 in response to the feedback information.

The light point beam received at the receivingoptical fiber device7 may be transmitted through the receivingoptical fiber device7 to anoutput port8. Theoutput port8 may be used to connect the receivingoptical fiber device7 to a transmission optical fiber device (not shown). In one embodiment of the present invention, theoptical communication system2 may be a packet switching device such as a network switch or a router, as illustrated in FIG.1. Thepacket switching device2 may include aprocessor202, aphysical interface card204, and arouting engine206. Thephysical interface card204 may receive signals representing a plurality of packets at one of a plurality of input/output ports8210212214. Thephysical interface card204 may forward the plurality of packets to arouting engine206. Therouting engine206 may receive the plurality of packets, decide the next step for the plurality of packets, and transmit the plurality of packets to a selected I/O port8 of the plurality of I/O ports on thephysical interface card204, wherein theselected port8 of the plurality of I/O ports8210212214 on thephysical interface card204 receives the plurality of packets. Theselected port8 of the plurality of I/O ports8210212214 may utilize optical communication technologies so the electrical signals may need to be converted to optical signals. The pointlight source driver5 may assist in converting electrical signals to optical signals by providing an output signal to the point light source representing the optical signals to transmit to represent the received plurality of packets. The point light source3 may transmit a point light beam to theoptical subassembly6, and theoptical subassembly6 may provide the incident light ray to thephotodetector4 to monitor the average output power and the extinction ratio of the point light source3. As discussed previously, thephotodetector4 may utilize the information from the incident ray to send average output power and extinction ratio data to the pointlight source driver5 which controls the output of the point light source3.

FIG. 2 illustrates an optical subassembly according to an embodiment of the present invention. Theoptical subassembly20 may include afirst side22, asecond side24, and athird side26. Theoptical subassembly20 may be composed of an optically transparent plastic, for example. Apoint light source10 may transmit a collimatedlight beam12 through thethird side26 of theoptical subassembly20. Anaspheric lens100 may be attached to the exterior surface of thethird side26 of theoptical subassembly20. The focal length of thelens100 may be selected to produce an image spot with a numerical aperture matching the numerical aperture of anoptical fiber core40. The chief ray of the collimatedlight beam12 may enter the axis of symmetry of theaspheric lens100.

The collimatedlight beam12 may travel through theaspheric lens100 andthird side26 of the optical subassembly to an interior surface of afirst side22 of theoptical subassembly20 where it is completely reflected and becomes aconvergent light beam14. The interior surface of thefirst side22 of theoptical subassembly20 may be a total internal reflection (TIR) surface, which means the collimatedlight beam12 is totally reflected to create theconvergent light beam14. The convergent light beam may travel through asecond side24 of theoptical subassembly20 and into a receivingoptical fiber device35. The receivingoptical fiber device35 may include anoptical fiber core40 andoptical fiber cladding30. In the receivingoptical fiber device35, theconvergent light beam14 may be transmitted only into theoptical fiber core40. Theoptical fiber cladding30 may enclose theoptical fiber core40 in relation to theoptical subassembly20.

Analignment ferrule50 may be used to precisely position theoptical fiber device35 at the focal point of theoptical subassembly20. Thealignment ferrule50 may be attached to theoptical subassembly20. Alternatively, thealignment ferrule50, theoptical subassembly20, and theaspheric lens100 may be manufactured as one part to minimize alignment inaccuracy. In this embodiment, theoptical subassembly20, thealignment ferrule50, and theaspheric lens100 may be made by an injection molding process using, for example, polycarbonate, polyolefin, or polyethylimide.

FIG. 3 illustrates a second side of an optical subassembly and the receiving optical fiber device according to an embodiment of the optical communication system. Thesecond side24 of theoptical subassembly20 may be aligned with theoptical fiber device35 in a manner to create anair gap70 between a section of thesecond side24 of theoptical subassembly20 and a section y of theoptical fiber device35. Illustratively, the air gap may be a wedge-shaped air gap. For example, as illustrated inFIG. 3, the wedge-shapedair gap70 may be of a length that is equal to the width of the optical fiber core and two sections x of theoptical fiber cladding30.

The creation of the wedge-shapedair gap70 may create anincident ray16 of theconvergent light beam14 in accordance with Fresnel reflection based on an index of refraction mismatch between air and the plastic of which theoptical subassembly20 is composed. In one embodiment, theincident ray16 of theconvergent light beam14 may be reflected almost 180 degrees relative to theconvergent light beam14. The angle between the incident ray and the convergent light beam is dictated by the angle of the wedge-shapedair gap70.

FIG. 4 illustrates a path of an incident ray of the convergent light beam and a path of a reflected light beam according to an embodiment of the optical communication system. Theincident ray16 of theconvergent light beam14 may be directed to the interior surface of thefirst side22 of theoptical subassembly20. Because the interior surface of thefirst side22 of theoptical subassembly20 may be a TIR surface, theincident ray16 of the reflected light beam may completely reflect off the interior surface of thefirst side22 of theoptical subassembly20 and become areflected light beam18, as illustrated in FIG.4.

The reflectedlight beam18 may travel through athird side26 of theoptical subassembly20. Theaspheric lens100 may be attached to the exterior surface of thethird side26 of theoptical subassembly20. Thelens100 may focus the reflectedlight beam18 onto aphotodetector80. Thelens100 may bring the reflectedlight beam18 to a focus so that the reflectedlight beam18 is brought to a relatively large fluence. The focus of the reflectedlight beam18 may align with the receiving area of thephotodetector80 to maintain the relatively large fluence. Because the reflected light beam maintains a relatively large fluence, asmall area photodetector80 may be used. This configuration may allow thephotodetector80 to track the high speed modulation of the reflectedlight beam18. Thephotodetector80 may receive the reflectedlight beam18 and determine the average output power of and the extinction ratio of the reflectedlight beam18, which should be equivalent to the average output power and the extinction ratio of the collimatedlight beam12 from the point light source10 (see FIG.2). Thephotodetector80 may provide this information to the point light source driver5 (seeFIG. 1) in order to correct any changes that may have occurred in the point light source's output power or extinction ratio due to age or change in temperature operating characteristics.

FIG. 5 illustrates a multi-channel wavelength division multiplexer with fiber optic input according to an embodiment of an optical communication system. The multi-channelwavelength division multiplexer100 may utilize a plurality of thin film filters (TFFs)102,104,106, and108 in a “zig-zag” scheme to perform channel separation. The multi-channel wavelength division multiplexer, or transmitter,100 may include anoptical subassembly120, aglass plate130, a plurality ofTFFs102104106 and108, a plurality of point light sources140 (only one shown), a firstaspheric lens150, a plurality of secondaspheric lenses142144146 and148, a plurality of focusingoptical subassemblies152154156 and158, and a plurality of photodetectors180 (only one shown). To simplify description, only onepoint light source140 and onephotodetector180 operation are illustrated. Light beams from pointlight source140 may be collimated, re-directed into a zig-zag optical path, as illustrated byFIG. 5, and finally coupled into anoptical fiber core180. In an embodiment, the pointlight source140 may be located in a position below theoptical subassembly120 and the plurality of focusingoptical subassemblies152154156 and158.

The plurality of point light sources140 (rest not shown) and the plurality ofTFFs102104106 and108 may have non-overlapping passbands, with each passband centered at the emitting wavelength of the corresponding pointlight source140.

A pointlight source140 may be positioned beneath a secondaspheric lens148. InFIG. 5, only one light beam (solid line), onepoint light source140, one reflected light beam (dotted lines), and onephotodetector180 are shown for clarity. The light beam emitted from the pointlight source140 may pass through its corresponding secondaspheric lens148. A focusingsubassembly158 corresponding to the secondaspheric lens148 redirects the collimated light beam into theglass plate130. The plurality ofTFFs102104106 and106 may be attached to a bottom surface of theglass plate130 as illustrated in FIG.5. The light beam may pass through the correspondingTFF108 because the emitting wavelength of the pointlight source140 corresponds to the passband ofTFF108. The rest of the plurality of point light sources pass through the remaining correspondingTFFs102104106108, respectively, in a similar fashion. Because the plurality ofTFFs102104106108 have non-overlapping passbands, different wavelength components are extracted from the plurality of collimated light beams from the plurality of point light sources. Inside theglass plate130, the collimated light beam from point light source140 (solid line) may travel in a “zig-zag” optical path while being reflected by the HR coatedsurface132 and the remainingTFFs102104 and106. The remainingTFFs102104 and106 may not allow the collimated light beam from pointlight source140 to pass because the emitting wavelength of the pointlight source140 does not correspond to the passband of the remainingTFFs102104 and106. After leaving theglass plate130, the collimated light beam may be coupled into anoptical subassembly120 through the firstaspheric lens150.

The collimated light beam may be directed through theaspheric lens150 and through athird side200 of theoptical subassembly120 to an interior surface of afirst side204 of theoptical subassembly120. The interior side of thefirst side204 may be a TIR surface. The collimated light beam may reflect off thefirst side204 and become a convergent light beam which is directed off thefirst side204 of theoptical subassembly120 through asecond side202 of theoptical subassembly120 and into theoptical fiber core190. Theoptical subassembly120 may be configured so that where thesecond side202 of the optical subassembly connects to theoptical fiber device190 an air gap may exist. (not shown). In one embodiment, a wedge-shaped air gap creates an incident ray (dotted line) of the convergent light beam in accordance with Fresnel reflection based on an index of refraction mismatch between air and the plastic of which theoptical subassembly120 is composed. The incident ray of the convergent light beam reflects back towards the interior surface of thefirst side204. The interior surface of thefirst side204 reflects the incident ray of the convergent light beam. The reflected light beam may pass through thethird side200 of theoptical subassembly120 and the firstaspheric lens150 to theglass plate130. The reflected light beam may travel in a “zig-zag” pattern alternately reflecting off the HR coatedsurface132 and the plurality ofTFFs102104 and106 (whose passband does not equal the emitting wavelength of the pointlight source140 which originally generated the reflected light beam). If the reflected light beam is of the wavelength that is allowed to pass through the corresponding TFF, in thiscase TFF108, the reflected light beam (dotted line) travels through the associatedTFF108, the associated focusingoptical subassembly158, and the associated secondaspheric lens148. The associatedaspheric lens148 focuses the reflected light beam onto the viewing portion of thephotodetector180, which monitors the output power and the extinction ration of the pointlight source140.

FIG. 6 shows a complete transceiver module according to an embodiment of the present invention. The complete transceiver module includes a dualfiber optic connector610, an injection-moldedoptical assembly650, a printed circuit board (PCB)630, and a metal shield for minimizing electromagnetic interference (not shown).Optical fibers10a,10bare connected to thedual fiber connector610. One optical fiber in thedual connector610 is for the receiver, and the other one is for the transmitter. As described, theoptical assembly650 is preferably a one-piece injection-molded optical subassembly with aconnector housing600. Thedual fiber connector610 slides into theconnector housing600. ThePCB630 is aligned with the one-piece injection-molded optical subassembly in theoptical assembly650. On thePCB630, there are laser sources, photodetectors, chips for processing electrical signals, other circuitry, etc. To aid the alignment, a ledge structure is provided in a plane parallel to the plane tangential to, and passing through, the apex of the aspheric lenses of the collimating and optical subassemblies in theoptical assembly650. The ledge structure allows thePCB630 to be inserted and to be parallel to the aspheric lenses within a few microns of tolerance.

While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of embodiments of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (16)

1. An optical communication system, comprising:

a receiving optical fiber device including an optical fiber core and optical fiber cladding;

a point light source to generate and transmit a point light beam; and

an optical subassembly adjacent to the receiving optical fiber device, including

an interior surface of a first side to receive the point light beam and to create a convergent light beam, and

a second side that the convergent light beam travels through to the receiving optical fiber device, wherein an exterior surface of the second side is positioned to create a wedge-shaped air gap to create an incident ray of the convergent light beam, the incident ray of the convergent light beam is directed to the interior surface of the first side of the optical subassembly to create a reflected light beam, and the reflected light beam travels through a third side of the optical subassembly, and

the third side of the optical subassembly includes a lens attached to an exterior surface of the third side of the optical subassembly to direct the reflected light beam to a photodetector and to direct the point light beam to the interior surface of the first side of the optical subassembly.

2. The optical communication system ofclaim 1, wherein the photodetector is a small aperture photodiode having a small capacitance to allow resolution of high speed amplitude modulation of the point light beam.

3. The optical communication system ofclaim 2, wherein the small aperture photodiode and the point light source are mounted on a common substrate.

4. An optical subassembly, comprising:

an interior surface of a first side to create a convergent light beam; and

a second side that the convergent light beam travels through, wherein the second side includes a wedge-shaped air gap to create an incident ray of the convergent light beam, the incident ray is directed to the interior surface of the first side to create a reflected light beam, and the reflected light beam travels through a third side of the optical subassembly, and

the third side of the optical subassembly includes a lens attached to an exterior surface of the third side of the optical connector to direct the reflected light beam to a photodetector and to collimate a point light beam to the interior surface of the first side of the optical subassembly.

5. The optical subassembly ofclaim 4, wherein the photodetector is a small aperture photodiode having a small capacitance to allow resolution of high speed amplitude modulation of the point light source output.

6. The optical subassembly ofclaim 5, wherein a point light source generating the point light beam and the small aperture photodiode are mounted on a common substrate.

7. A method of monitoring a point light source output power and extinction ratio, comprising:

creating a convergent light beam by reflecting a collimated light beam wave off an interior surface of a first side of an optical subassembly;

creating an incident ray of the convergent light beam by including a wedge-shaped air gap on a second side of the optical subassembly,

creating a reflected light beam by directing the incident ray of the convergent light beam to the interior surface of the first side of the optical subassembly, the reflected light beam traveling through a third side of the optical subassembly; and

further including directing the reflected light beam to a photodetector through a lens attached to an exterior surface of the third side of the optical subassembly, and directing a light beam from a point light source through the lens to the interior surface of the first side of the optical subassembly.

8. The method ofclaim 7, wherein the photodetector and the point light source are mounted on a common substrate.

9. A packet transfer/switching device, comprising:

a processor;

a physical interface card to receive a plurality of packets and to forward the plurality of packets including

a plurality of input/output (I/O) ports; and

an optical subassembly including

an interior surface of a first side to create a convergent light beam;

a second side that the convergent light beam travels through, wherein the second side includes a wedge-shaped air gap to create an incident ray of the convergent light beam; and

a routing engine to receive the plurality of packets, to decide the next step for the plurality of packets, and to transmit the plurality of packets to a selected I/O port of the plurality of I/O ports on the physical interface card, wherein the selected port of the plurality of I/O ports on the physical interface card receives the plurality of packets, utilizes the optical subassembly to assist in converting the plurality of packets to optical signals for transmission and transfers the optical signals representing the plurality of packets to a second packet/switching device.

10. The device ofclaim 9, wherein the packet switching device is a network switch.

11. The device ofclaim 9, wherein the packet switching device is a router.

12. An integrated optical assembly, comprising:

at least two point light sources to generate at least two point light beams each having a corresponding emitting wavelength;

at least two corresponding focusing optical subassemblies, the at least two focusing optical subassemblies being aligned along a common axis, receiving the at least two point light beams, and creating at least two collimated point light beams;

an optically transparent block that receives the at least two collimated point light beams from the at least two corresponding focusing optical subassemblies, wherein the at least two collimated light beams travel in a zig-zag fashion through the optically transparent block and exits the optically transparent block; and

an optical subassembly which receives the at least two collimated light beams and creates at least two reflected light beams utilizing an air gap on a second side of the optical subassembly, wherein the at least two reflected light beams enter the optically transparent block, travel through the optically transparent block, exit the optically transparent block at one of at least two thin-film filters having a passband corresponding to the emitting wavelength of one of the at least two point sources, travel through one of the at least two corresponding focusing optical subassemblies, wherein the at least two reflected light beams are focused onto a photodetector.

13. The integrated optical assembly ofclaim 12, where the optical subassembly includes

an interior surface of the first side to reflect the at least two collimated light beams and to create at least two convergent light beams; and

a second side that the at least two convergent light beams travels through, wherein the second side includes the air gap to create at least two incident rays of the at least two convergent light beams.

14. The integrated optical assembly ofclaim 13, wherein the at least two incident rays of the at least two convergent light beam are directed toward the interior surface of the first side to create at least two reflected light beams.

15. The integrated optical assembly ofclaim 14, wherein the at least two reflected light beams travel through a third side of the optical subassembly.

16. The optical subassembly ofclaim 15, wherein the third side of the optical subassembly includes a first aspheric lens attached to an exterior surface of the third side of the optical subassembly to direct the at least two reflected light beams to the optically transparent block.

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